Production of concentrated aqueous solutions of ferric chloride

ABSTRACT

Concentrated aqueous solutions of ferric chloride, directly useful as flocculating agents in a variety of water treatments, are prepared by (a) digesting iron with a dilute aqueous solution of hydrochloric acid until such acid has essentially been consumed and thereby producing an aqueous solution of ferrous chloride; (b) reacting chlorine with such aqueous solution of ferrous chloride, in the presence of a recycled aqueous solution of ferric chloride; (c) decompressing the step (b) reaction product solution to vaporize water therefrom, thereby concentrating same; and (d) recycling a fraction of the decompressed liquid phase to step (b) and recovering remaining fraction as final product aqueous solution of ferric chloride.

This application is a continuation of application Ser. No. 07/672,772,filed Mar. 20, 1991, now abandoned, which is a continuation ofapplication Ser. No. 07/345,241, filed May 1, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the preparation of ferric chloride fromdilute hydrochloric acid, and, more especially, to the preparation of35% to 45% by weight aqueous solutions of ferric chloride.

2. Description of the Prior Art

The above aqueous solutions of ferric chloride are known to be useful asflocculating agents in water treatment. Compare, for example,Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, volume24, pages 394-396 (1984) and volume 10, page 498 (1980).

The simplest process for the preparation of ferric chloride solutionsentails digesting iron with concentrated hydrochloric acid; a solutioncontaining approximately 36% by weight of ferrous chloride (FeCl₂) isthus obtained, and this is chlorinated to produce an aqueous solution offerric chloride (FeCl₃) analyzing, by titration, at approximately 41% byweight. This 41% solution can be directly employed as a flocculatingagent, and is the usual commercial form. A concentrated solution ofFeCl₂ must be produced because FeCl₂ and FeCl₃ can undergo a partialhydrolysis over the course of concentration by evaporation, to give HCl.The presence of HCl in FeCl₃ is a particular problem in water treatment.The above process also requires the use of concentrated hydrochloricacid.

U.S. Pat. No. 4,066,748 describes a process for preparing ferricchloride beginning with a solution of FeCl₂ emanating from a descalingbath. This process requires both a concentration of the ferrous chlorideas well as a two-step chlorination.

U.S. Pat. No. 3,682,592 describes a process similar to that of the '748patent, but in which the ferrous chloride solution is contacted withoxygen.

Serious need exists in this art for a simple process for preparingferric chloride from dilute hydrochlorine acid. A notable solution ofhydrochloric acid is the water/HCl azeotrope, at a concentration of20.5% by weight at atmospheric pressure. However, to produce ferrouschloride solutions containing a least 36% by weight FeCl₂, it isnecessary to begin with an approximately 24% by weight acid solution.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofa simple process for the preparation of ferric chloride from dilutehydrochloric acid, which novel process permits the use of acid solutionshaving concentrations less than azeotropic and which avoids having toconcentrate the solutions of HCl, and/or of ferrous chloride, and/or offerric chloride.

Briefly, the present invention features the preparation of ferricchloride from dilute hydrochloric acid, comprising:

(a) contacting a dilute aqueous solution of hydrochloric acid with ironuntil the hydrochloric acid has been consumed;

(b) chlorinating the resulting solution to produce an aqueous solutionof ferric chloride;

(c) decompressing or reducing the pressure over the reaction productproduced in step (b); and

(d) recycling a fraction of the decompressed liquid phase to step (b),while recovering the remaining fraction as final product aqueoussolution of ferric chloride.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE of Drawing is a schematic/diagrammatic illustration of theprocess/apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

More particularly according to the present invention, the iron used instep (a) is either iron metal or iron oxides, or a mixture of metalliciron and iron oxides. It is convenient to employ iron cuttings, such aswastes from the machining of steel components, turnings resulting fromthe boring of steel components, or the residues from stampingoperations. This recovered iron is oxidized to a greater or lesserextent.

The dilute hydrochloric acid solution is advantageously an aqueoussolution of hydrochloric acid containing from 15% to 25% by weight ofhydrochloric acid. The use of a higher concentration is also within thescope of the present invention, but it would be more simple in this caseto employ a conventional prior art process. This invention isparticularly applicable for the so-called residual acids which cannot beconcentrated beyond the azeotropic composition. In general, these acidsare at a concentration ranging from 15% by weight to the azeotrope. Theuse of an acid containing less than 15% by weight of hydrochloric acidtoo is within the scope of the invention, but it would be far simpler tofirst concentrate this acid to above 15% by conventional distillation.The dilute solution of the hydrochloric acid may also contain ferrouschloride and possibly also a minor amount of ferric chloride.

The step (a) may be carried out in any manner, but the operation isadvantageously carried out at atmospheric pressure in a reaction vesseland under conditions such that a stoichiometric excess of iron is alwayspresent, to ensure that no hydrochloric acid remains in the ferrouschloride solution which results at the termination of this step (a). Thereaction between the iron and the hydrochloric acid is complete andstoichiometric. Although the solution containing the hydrochloric acidmay be at any temperature, a solution is advantageously employed havinga temperature ranging from 20° to 50° C. and preferably from 35° to 45°C. The residence time of the acid may be of any length, butadvantageously it ranges from 2 minutes to 30 hours, and preferably from3 to 15 hours. It is also within the scope of this invention to add instep (a), in addition to the solution of dilute hydrochloric acid, anaqueous solution containing both hydrochloric acid and ferrous chloride,or containing ferrous chloride alone, such added solution comprising,for example, one emanating from a descaling bath. It is preferable thatthe amount of such added solution constitute not more than 20% and,preferably, not more than 10% by weight of the solution of residualhydrochloric acid. Using only a solution emanating from a descalingoperation and which contains both FeCl₂ and HCl is also within the scopeof the invention, but such a solution must not contain an excess ofheavy metals, as this would present certain difficulties if the finalproduct solutions were intended for the preparation of drinking water.

Prior to chlorinating the solution resulting from step (a), it isadvantageous to either filter or decant it.

Step (b) is carried out by contacting the solution of ferrous chlorideproduced in step (a) with chlorine and a solution of ferric chloride.Liquid or gaseous chlorine, or a fluid containing chlorine, may thus beemployed. The contacting may be carried out in any manner; it sufficesonly to ensure intimate contact between the chlorine and the ferrouschloride. The apparatus employed may have a number of stages operatingin parallel or in series, and charged either cocurrently orcountercurrently, or in any combination thereof.

For example, a stirred reactor or a reactor comprising a distillationcolumn or absorption column, may be employed. Advantageously, a columnequipped with contacting means such as plates or packing rings or with anumber of these means may be employed. The ferrous chloride solution andthe ferric chloride solution are charged into the top of the column andthe chlorine is countercurrently introduced at the base of this column.A minor amount of chlorine is collected at the top of this column, if astoichiometric excess thereof has been employed (relative to the amountof ferrous chloride), together with any gases which may have accompaniedthe chlorine, as well as a minor amount of water vapor which has beenentrained by its vapor pressure. An aqueous solution of ferric chlorideis recovered from the base of the column. It is also within the scope ofthe present invention to introduce the ferrous chloride and ferricchloride solutions at a number of points along the column. Likewise, thechlorine may be introduced at a number of points along the column.

The ferric chloride solution produced is next decompressed and afraction of the liquid phase collected during this decompression isrecycled to the step (b), while the remaining fraction is recovered asfinal product aqueous solution of ferric chloride. During thedecompression, a gaseous phase consisting essentially of water vapor isobtained.

The chlorination of ferrous chloride to ferric chloride is complete andstoichiometric. Advantageously, a ferrous chloride residence time of atleast 10 seconds, and preferably less than 4 hours, is observed. It isnot necessary to chlorinate all of the FeCl₂ ; the specifications forFeCl₃ sometimes permit from 0.1 to 1% by weight of FeCl₂ in the ferricchloride solution. It too is within the scope of this invention toemploy a stoichiometric excess of the chlorine.

The temperature of the ferrous chloride solution produced in step (a)advantageously ranges from 40° to 100° C., and preferably from 60° to90° C. The temperature of the ferric chloride solution which is recycledand which results from the decompression in step (c) depends upon thetemperature at the outlet of step (b) and the pressure at which thedecompression is carried out. The temperature of the recycled ferricchloride at the inlet to step (b) advantageously ranges from 50° to 90°C., and preferably from 55° to 80° C. The temperature of the ferrouschloride produced in step (a), and the temperature of the ferricchloride recycled to the step (b) may be modified, for example with theaid of heat exchangers. Preferably, these solutions are used withouttheir temperatures being adjusted. The temperature of the outlet ferricchloride solution depends essentially on the inlet temperatures of thesolutions and on the amount of ferrous chloride to be converted intoferric chloride, but also on whether the apparatus utilized to performthe step (b) is adiabatic, isothermal, etc.

An adiabatic apparatus is preferred; the outlet temperature of thesolution upon completion of this step (b) advantageously ranges from 60°to 95° C., and preferably from 65° to 90° C. Although the operation maybe carried out at any pressure, it is simpler to conduct the step (b) atatmospheric pressure, or at a pressure close to atmospheric. Thissolution is decompressed at a pressure ranging from atmospheric pressureto a pressure of from 0.3 to 0.05 bar absolute. The decompression iscarried out in a vessel of any type; it suffices merely that it have ageometry enabling separation of the gaseous phase and the liquid phaseresulting therefrom. The vacuum is provided, for example, by a pump of asteam ejector.

In a static mode, the amount of ferric chloride constituting the outputcorresponds, in the number of moles, to the amount of ferrous chlorideintroduced into step (b). The amount of ferric chloride solutionrecycled to step (b) advantageously ranges from 5 to 10 times the amountof the ferric chloride solution constituting the output in this mode.

In another embodiment of the invention, the ferric chloride solutionproduced in step (b) may also be heated prior to being decompressed.

The heat exchangers which may be incorporated in the process loop toadjust the temperature of the ferrous chloride solution introduced intostep (b), the temperature of the ferric chloride solution produced insaid step (b), and the temperature of the recycled ferric chloridesolution, may also be used to extract the heat energy in the case wherethe reaction becomes excessively exothermic, for example if theconcentration of hydrochloric acid drifts to higher values.

In a preferred embodiment of the invention, and provided that thetemperature levels are compatible, the heat energy from the productionof ferric chloride may be transferred to the ferrous chloride feedstreamsolution for step (b).

Apparatus suitable for carrying out the process of the invention isshown in the FIGURE of Drawing. For the sake of clarity, the pumps, thevalves, etc., have not been shown.

Step (a) is carried out in the vessel 1. The iron is introduced by inletline 5 and the solution of hydrochloric acid through inlet line 6. Asolution of ferrous chloride is thus produced which outlets via line 7and is, filtered in filter 2. Column 3 is charged with a stream offerrous chloride through line 8 and with a solution of ferric chloridethrough line 9. This column 3 is also charged with a stream of gaseouschlorine via line 14. The resulting ferric chloride solution exitsthrough line 10 and is decompressed in the vessel 4. The decompressedferric chloride solution is withdrawn through line 12 and is dividedinto a final product recovery via outlet 13 and a fraction which isrecycled via line 9. The decompression vessel 4 is connected via theline 11 to a steam ejector, not shown. The inerts and any unreactedchlorine are removed from the column 3, via outlet 15.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

EXAMPLE 1

An apparatus as shown in the FIGURE of Drawing was used, in which thecolumn 3 was constructed of glass, having an inner diameter of 0.35 mand a packing height of 10 m.

A solution containing 170 kg/h of hydrochloric acid in 680 kg/h of waterwas introduced through inlet 5. A quantity of iron turnings greatly instoichiometric excess relative to the amount of HCl had been introducedbeforehand via line 6. The ferrous chloride solution was filtered andthen introduced into the column through the line 8; its temperature was70° C. A flow of 82.7 kg/h of gaseous chlorine was introduced throughline 14, together with 5 kg/h of inerts, which exited the system viaoutlet 15. The column 3 was operated at a pressure of 1.1 bars absolute.The ferric chloride solution, at 82° C., was conveyed by the conduit 10to the decompression vessel 4, where it was decompressed to 0.2 bars;its temperature decreased to 70° C. A final product solution of 379 kg/hof FeCl₃ diluted in 545 kg/h of water was recovered via outlet 13. 2,272kg/h of FeCl₃ diluted in 3,269 kg/h of water were recycled via line 9.

EXAMPLE 2

The procedure of Example 1 was repeated, but the solution ofhydrochloric acid was more dilute, i.e., 170 kg/h of HCl in 963 kg/h ofwater. The temperature of the stream in line 8 was 60° C. The ferricchloride solution transferred through line 10 was at a temperature of84° C. It was heated to 93° C. prior to decompression in vessel 4 to0.12 bars. After decompression, the same concentrated solution as inExample 1 was recovered via outlet 13, but, at 57° C., an amount of3,032 kg/h of FeCl₃ diluted with 4,360 kg/h of water was recycled vialine 9. This solution was heated to 75° C. prior to the recycle into thecolumn 3.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A process for the preparation of an aqueoussolution of ferric chloride, comprising (a) digesting iron with a diluteaqueous solution of hydrochloric acid until said acid has beenessentially consumed thereby producing an aqueous solution of ferrouschloride; (b) reacting chlorine with said aqueous solution of ferrouschloride, in the presence of a recycled aqueous solution of ferricchloride to form a reaction product solution; (c) reducing pressure overthe step (b) reaction product solution to vaporize water therefrom andconcentrate said reaction product solution to obtain a concentratedliquid phase containing at least 35% by weight ferric chloride; and (d)recycling a fraction of the concentrated liquid phase to step (b) andseparating the remaining fraction of the concentrated liquid phase as afinal product of concentrated aqueous ferric chloride solution.
 2. Theprocess as defined by claim 1, wherein the concentration of said diluteaqueous solution of hydrochloric acid is between 15% by weight and thewater/HCl azeotropic composition thereof.
 3. The process as defined byclaim 2, wherein the amount of recycled liquid phase ranges from 5 to 10times the amount of separated fraction of final product solution.
 4. Theprocess as defined by claim 1, wherein said concentrated aqueous ferricchloride solution has a concentration of 35% to 45% by weight.
 5. Theprocess as defined by claim 1, wherein the pressure in step (c) is apressure of between about atmospheric pressure and about 0.3 barabsolute.
 6. The process as defined by claim 1, wherein the pressure instep (c) is a pressure of between about atmospheric pressure and about0.05 bar absolute.
 7. The process for the preparation of an aqueoussolution of ferric chloride according to claim 1, wherein said diluteaqueous solution of hydrochloric acid contains from 15% to 25% by weightHCl.
 8. The process for the preparation of an aqueous solution of ferricchloride according to claim 1, wherein said aqueous solution of ferrouschloride in step (a) has a temperature of between 40° C. and 100° C. 9.The process for the preparation of an aqueous solution of ferricchloride according to claim 1, wherein said fraction of the concentratedliquid phase which is recycled to step (b) has a temperature of between50° C. and 90° C.